i own both quake4 and cod2. and i dont touch q4 since i got cod2. the single player on both are good. but cod2 is overall better (alot more depth, alot more intesne imo)
as for multiplayer, cod2 isnt as bad as people make it out to be. sure i get into the odd laggy game, but lately ive been host and it doesnt lag at all
even when im not host, laggy games seem to only be about 1/4 of my time. if it lags quit and go into another.
i guarantee you youd enjoy cod2 more then q4. the controls are beter, and its just overall alot of fun